1. Field of the Invention
The present invention relates generally to methods and apparatus for testing integrated circuits, and more particularly but not exclusively to test probes.
2. Description of the Background Art
Probe cards are employed in integrated circuit (IC) testing. A probe card includes a substrate that supports multiple probes for contacting corresponding pads on an integrated circuit die (also referred to as a “chip”). Generally speaking, the probe card provides an interface between a test equipment (e.g. an automated test equipment or “ATE”) and the integrated circuit die under test (DUT).
FIG. 1 schematically illustrates an example array of conventional probes 11. Each probe 11 typically has a supported portion (not shown) attached to a probe card substrate and an unsupported portion 13 that flexes upon contact with a die pad. Only the unsupported portions 13 are shown in FIG. 1 for clarity of illustration. The length of an unsupported portion 13 of each probe 11 is shown as L, the width of each probe 11 is shown as w, and the pitch between probes 11 is shown as P. In FIG. 1, each probe 11 has an unsupported portion 13 with uniform cross-section (rectangular in FIG. 1). Each unsupported portion 13 includes a tip 12. The tips 12, which may be integral or separately attached, physically contact corresponding pads on a die under test to make an electrical connection.
The probes 11 are pre-stressed to curve or extend outwardly from a planar surface of a substrate (e.g. printed circuit board) of a probe card. The probes 11 can be formed and released, for example, using Micro-Electromechanical Systems (MEMS) fabrication techniques. When the wafer containing the die is raised or moved toward the probe card beyond the point at which the first pads on the die first come into contact with the probes 11, the probes 11 flex so as to allow the tips 12 of the other probes 11 to contact pads on the die, thereby compensating for small variations in planarity or parallelism between the probe card and the die on the surface of the wafer. The movement of the wafer past the point at which the tips 12 contact pads on the die and the resultant flexing of the probes 11 also cause the tips 12 to scrub across their respective pads, thereby removing oxide buildup on the pads and improving electrical connection between the probe card and the die.
One problem with the probes 11 is that they suffer from low stiffness. Another problem is that as the probes 11 flatten against the substrate of the probe card, van der Waals force can cause the probes 11 to be held against the surface of the probe card substrate, possibly degrading the probe card or rendering it unusable.
One approach to addressing the above problems is to form the probes with unsupported portions projecting over one of a number of cavities formed on the surface of the probe card substrate. While this approach reduces the potential for van der Waals forces, it does not address the low stiffness of rectangular beams formed using MEMs techniques and it can decrease the force applied to the die and test.
FIG. 2 schematically shows an example conventional MEMS-based probe with uniform cross-section unsupported portions under different load conditions. In the example of FIG. 2, the profile 21 represents a side view of the probe under no load condition, the profile 22 represents a side view of the probe under medium load condition, and the profile 23 represents a side view of the probe under maximum load condition. In FIG. 2, the vertical axis represents height and the horizontal axis represents distance. The height at unit 0 represents the surface of the probe card substrate; the die under test (not shown) would be at positive units of height above 0. As load is applied to the probe upon contact with a pad of the die under test, the probe flexes towards the probe card substrate as illustrated by the transition from profile 21 to profile 22 then to profile 23.
In FIG. 2, the load applied to the probe is uniform, and the profile of the probe in unloaded, pre-stressed condition (profile 21) is cylindrical. From FIG. 2, it can be seen that for an end loaded cantilever beam probe with uniform cross-section (e.g. probe 11), the moment (M(x)) is greatest at base and is given by the following equation:
                              M          ⁡                      (            x            )                          =                              (                          FL              -              Fx                        )                    w                                    (                  Eq          .                                          ⁢          1                )            where F is the force applied at the tip of the probe, L is the length of the unsupported portion of the probe, x is the distance along the length and w is the width of the probe.
The deflection (y(x)) of a cantilever beam probe with uniform cross-section is cubic and given by the following equation:
                              y          ⁡                      (            x            )                          =                              F                          6              ⁢                                                          ⁢              BI                                ⁢                      (                                          3                ⁢                                  x                  2                                ⁢                L                            -                              x                3                                      )                                              (                  Eq          .                                          ⁢          2                )            where B is the modulus I is the moment of inertia, F is the force applied at the tip of the probe, L is the length of the unsupported portion of the probe, and x is the distance along the length.
As illustrated by the profile 23, the probe can deflect below the probe card substrate under high load conditions. If the probe is on a probe card substrate that has no cavity below the unsupported portion of the probe, the probe will come into contact with the probe card substrate. This may damage the probe or cause the probe to stick to the probe card substrate and thus interfere with the test operation. If, however, the unsupported portion of the probe is over a cavity, the unsupported portion of the probe will curve into the cavity below the plane of the probe card substrate, undesirably further decreasing the force applied to the die under test and consequently reducing the scrubbing action of the tip of the probe on the pad of the die. The incorporation of cavities also increases the cost of fabricating the probe card.